1. Field of the Invention
The present invention relates to a telephoto zoom lens, and, more particularly, to a telephoto zoom lens of a structure consisting of five lens groups, the telephoto zoom lens revealing a relatively compact size and reduced number of component lenses.
2. Related Background Art
Recently, there has been a desire for a telephoto lens of the type described above revealing a relatively compact size and reduced number of component lenses to have excellent imaging performance in its overall power variable region as well as revealing compact size and reduced cost with maintaining high zooming ratio. Therefore, a multiplicity of disclosures have been made, for example, in Japanese Patent Laid-Open No. 56-114919, U.S. Pat. No. 4,673,258 and the like, and have been put into practical use.
A telephoto zoom lens of an optical compensation type disclosed in the above-described Japanese Patent Laid-Open No. 56-114919 is constituted by four lens groups: a positive, a negative, a positive and a positive lens groups, that telephoto zoom lens exhibiting a characteristics in that each of the above-described lens groups has relatively weak refracting power. Therefore, the degree of freedom can be raised when the aberration is desired to be compensated so that the number of the component lenses can be reduced.
However, since a telephoto zoom lens of the type described above basically has relatively weak refractive power, the variable power ratio (zoom ratio) cannot be easily raised. If the variable power ratio is forcibly raised, the distance in which each of the lens groups must move at the time of varying the magnification becomes too long. What is even worse, another problem arises in that the overall length of the lens system becomes too long. An optical compensation type zoom lens of the type described above usually encounters the above-described problem. In particular, the reason for the fact that the telephoto zoom lens of the type described above inevitably has long overall length at its wide angle end lies in that the telephoto ratio (the overall length of the lens system/focal distance of the lens system) at the wide angle end is raised due to the basic configuration of refracting power, that is, positive, negative, positive and positive refracting power.
Therefore, it might be considered feasible to employ means arranged to enlarge the refracting power of each of the lens groups for the purpose of reducing the size of the telephoto zoom lens.
If the refracting power of each of the lens groups is enlarged, an excellent effect can be obtained in that the distance of the movement of the lens necessary at zooming (varying the magnification) can be shortened while shortening the overall length of the lens. However, freedom in compensating the aberration becomes unsatisfactory, causing the aberration variation such as the lower aberration or the spherical aberration to be undesirably increased. Accordingly, if the degree of freedom in compensating the aberration is improved by increasing the number of the component lenses, the weight of the lens system becomes too heavy and the structure becomes complicated. Therefore, the overall cost cannot be reduced.
If the overall length is shortened by arranging the structure in such a manner that each of the lens group has large refracting power, the overall length cannot satisfactorily be shortened due to the above-described distribution of the refracting power arranged as: positive, negative, positive and positive refracting power Therefore, the overall length cannot be satisfactorily shortened. If the refracting power of each of the lens groups is enlarged, the variable power ratio can be raised and the degree of freedom in compensating the aberration becomes insufficient, causing the field curvature to be varied. In particular, the upper comatic aberration is varied excessively.
The telephoto zoom lens disclosed in U.S. Pat. No. 4,673,258 is constituted by four lens groups: positive, negative, positive and negative lens groups. Furthermore, each of the lens groups is arranged to have relatively weak refracting power so that the number of the component lenses is reduced. In addition, the distribution of the refracting power of each of the lens groups is arranged in such a manner that the telephoto ratio at the wide angle end can be reduced. Therefore, the overall length of the lens system can be shortened.
However, in the case where the telephoto zoom lens in the above-described disclosure is a high magnifying power type the variable power ratio of which exceeds 2.8, the variation of the spherical aberration, the field curvature and the comatic aberration becomes too large. In particular, a lower comatic aberration is generated at the wide angle end in the positive direction, causing the symmetry of the comatic aberration to be critically distorted.
Furthermore, the telephoto zoom lens of this type is, as shown in FIG. 2, arranged in such a manner that the second lens group of the negative variable power, the third lens group of the positive variable power and the fourth lens group of the negative power are disposed very close to one another at the telephoto end. Therefore, that telephoto zoom lens is substantially constituted by two lens groups: consisting of a forward lens group having positive refracting power and composed by the first lens group; and a rearward lens group having negative refracting power and composed by the second to the fourth lens groups.
However, since each of the component lenses of the rearward lens group (the second, the third and the fourth lens groups) are arranged to act in association with one another, the degree of freedom in correcting the aberration is lowered. As a result, it is very difficult to properly and simultaneously compensate the axial aberration and the aberration deviated from the axis.